1. Field of the Invention
The present invention relates to a brushless motor and an axial flow fan with the brushless motor for cooling a machine.
2. Background Discussion
FIG. 10 is a circuit diagram showing a drive-control circuit of a conventional brushless motor disclosed in, for instance, Japanese Unexamined Patent Publication No. 37895/1984. In FIG. 10, a reference numeral 1 designates a d.c. power source, a numeral 2 designates a resister, a numeral 3 designates a rotor position detecting element such as a hall element, i.e. a hall IC or the like which is adapted to detect a specified position during the rotation of a rotor with permanent magnets (not shown), a numeral 4 designates an amplifier, numerals 5, 6 designate stator coils, numerals 7, 8 designate driving transistors connected in series to the stator coils 5, 6 respectively, a numeral 9 designates an inversion circuit, a numeral 10 designates a phase-lock operation circuit which prohibits the actuation of the stator coil of a specified phase, a numeral 11 designates a transistor which is turned on at the time of phase-lock operation to thereby turn off the driving transistor 8, and a numeral 12 designates a start compensation circuit which turns on a transistor 13 when a stopping condition of the rotor is distinguished, and which turns off the actuation of the phase-lock operation circuit 10. A character A designates a coil driving circuit constituted by the amplifier 4, driving transistors 7, 8 and the inversion circuit 9.
The operation of the conventional drive-control circuit of the brushless motor will be described.
In a normal operational state, the phase-lock operation circuit 10 is switched to be a non-operation state, and the transistor 11 is turned off. When a specified position of the rotor is detected by the rotor position detecting element 3, a detection signal is produced at the detection element 3 and is amplified by the amplifier 4. The amplified signal actuates either the driving transistor 7 or the drive transistor 8 (through the inversion circuit 9), whereby a current is alternately supplied to the stator coils 5, 6, and a torque is produced in the rotor so that the rotor is continuously rotated.
In a phase-lock operation, the phase-lock circuit 10 is changed to an operational condition, whereby the transistor 11 is actuated. The actuation of the transistor 11 turns off the transistor 8, so that a current is intermittently supplied to only the stator coil 5, and the rotor is rotated in a low speed. In this case, when current supply to the stator coil 5 is stopped and the rotor stops, no torque is produced at the restarting time in the phase-lock operation, and the starting is not obtained. In this case, however, the output of the rotor position detecting element 3 does not change unless the rotor is not rotated. When a timer installed in the start compensation circuit 12 detects that a time during which the output of the rotor position detecting element 3 does not change, exceeds a predetermined time, the transistor 13 is actuated whereby the transistor 11 is turned off. During a current supply time to the stator coil 6, the drive transistor 8 is turned on, whereby the stator coil 6 is actuated. Then, the rotor starts to rotate by a torque provided by the actuation of the coil 6 unless there is no obstacle.
When the rotor is rotated, the output of the rotor position detecting element 3 varies, whereby the start compensation circuit 12 is reset, and the transistor 13 is turned off. Thus, it is possible to carry out the above-mentioned phase-lock operation.
In the conventional brushless motor having the abovementioned construction, the revolution speed of the rotor during the phase-lock operation is determined by the phase angle at which a current is prohibited from flowing, and it is impossible to change the revolution speed. Further, in the conventional brushless motor, it is necessary to use a start compensation circuit in order to certainly re-start the motor even in a phase-lock operation. In an application of the brushless motor to an axial flow fan, it was difficult to change the revolution speed depending on the temperature of an object to be cooled. Further, it was impossible to produce a signal indicative of an abnormal state such as overheating of the object to be cooled or the axial flow fan falling into a lock condition, such signal to be supplied to the main body side of the apparatus.